1. Field of the Invention
The present invention relates generally to a projection type image display apparatus, and more particularly to a projection type image display apparatus including a transmissive display panel such as a liquid crystal panel, having a plurality of picture elements arranged in a matrix. This type of display apparatus is particularly applicable to a large-screen projection TV (television), information display system, or the like.
2. Description of the Prior Art
In order to obtain parallel light rays, the projection type image display apparatus employs a parabolic reflecting mirror or an oval reflecting mirror. When a parabolic reflecting mirror is employed, the light source is placed at the focal point of the parabolic reflecting mirror so as to obtain parallel rays reflected from the parabolic mirror. The parallel rays are introduced through the transmissive display panel. When an oval reflecting mirror is employed, the light source is placed at a first focal point of the oval reflecting mirror, and the rays are focused on a second focal point, thereby allowing the rays to pass through the display panel as parallel rays. The transmissive display panel does not emit light rays but its transmissibility varies in accordance with a driving signal applied thereto. By modulating the intensity of light applied to the display panel, the pictures and characters are displayed.
For the transmissive display panels, display devices using a transmissive ceramic are known: for example, a liquid crystal panel, an electro-chromic display, a lead lanthanum zirconate titanate (PLZT). A liquid crystal panel is widely used in pocket-size television sets and word processors. Hereinafter, the liquid crystal display panel will be described as a typical example of the present invention.
In a liquid crystal display panel, commonly called a matrix type liquid crystal display panel, picture elements (openings) are arranged in a matrix in two directions perpendicular to each other and individually driven by driving voltages so as to change the optical characteristic of the liquid crystal. Thus, pictures and characters are displayed. A driving voltage can be applied to the individual picture elements by a simple matrix system or alternatively by an active matrix system under which a non-linear two-terminal element such as MIM (metal-insulating metal) or a three-terminal switching element such as TFT (thin film transistor) is disposed for each picture element.
Under the active matrix system the picture elements must be arranged with minimum pitches, which tend to reduce the ratio of an effective opening occupied by the picture elements to the total area, hereinafter referred to as the "aperture ratio". If all components of the display panel are reduced in size proportionally to the reduced pitches of picture elements, no change occurs in the aperture ration but owing to unavoidable irregularities in etching and alignment, the reduction in the sizes of metallic wiring to the electrodes, non-linear elements, TFT, etc. has a limit and they must be made to such a tolerance as to allow the irregular sizes. As the picture element pitches are minimized, the aperture ratio becomes low, and regeneration images become dark.
Japanese Laid-Open Patent Publications Nos. 60-262131, 61-11788, and 2-12224 disclose systems for increasing the aperture ratio, which are in common characterized by an arrangement in which an array of microlenses, such as fly eye lenses and lenticular lenses, are disposed on the light incident side or on the light incident and outlet sides of the display device in correspondence to each picture element. The light is focused in each picture element through the lens elements disposed on the light incident side.
Under these known systems the parallelism of rays is practically limited by the length of arc (i.e. the length of illuminating section) used as a light source, and the precision of the reflecting mirror, thereby making it difficult to focus light rays in the picture elements. In order to effectively focus light rays, an oval reflecting mirror is employed in the manner as shown in FIG. 3(a). As a light source an arc lamp 12 is disposed at a first focal point of the oval reflecting mirror 11 so as to enable the direction of the arc and the optical axis to be in accord with each other. If the arc lamp became a perfect point of illumination, the light rays therefrom would be focused at the second focal point of the oval reflecting mirror 11. In fact, however, they are not focused at the second focal point because the arc has a length and the reflecting mirror 11 inherently has an irregularity. The longer the arc becomes, the more the focusing spot diverges. The light rays passing through the second focal point become parallel by condenser lenses or any other optical means. However, the larger the focusing spot becomes, the less parallel the light rays become after passing through the condenser lenses.
When a parabolic reflecting mirror is employed, the arc lamp 12 is disposed at the focal point of the parabolic reflecting mirror 13 as shown in FIG. 3(b) so that the arc lamp 12 is disposed at the focal point of the parabolic reflecting mirror 13 so as to direct the arc in the same direction as that of the optical axis. In this case, when the arc of the arc lamp 12 becomes too long, the light rays tend to diverge with a relatively large angle .alpha., thereby resulting in the reduced parallelism of light rays from the source.
The parallel rays are focused on the picture elements by the microlenses disposed at the light incident side of the transmissive display panel. As shown in FIG. 4, a light having an angle of.+-..theta. is incident to the microlens 14 expands in a circle having a diameter of (2f.times.tan .theta.) on the display panel 15 disposed at a focal point (f) of the microlens 14. If the expansion exceeds beyond the picture element 15, the focusing spot fails to be within the picture element 15. In this way the advantages of the microlens are negated.
The known display system having an arc lamp disclosed such that the arc is generated in the same direction as that of the optical axis of the microlens has a disadvantage in that the electrodes of the arc lamp obstruct the passage of light, thereby producing an optical vacancy at the center of the illumination. This vacancy of illumination produces uneven brightness on the screen, thereby failing to reproduce uniformly bright image pictures and characters.